Dose-effect relations for pentobarbital and phenobarbital will be determined in rats using a behavioral discriminative motor control procedure. Using this technique, interaction and cross-tolerance between barbiturates and other agents will be evaluated as well. The development of barbiturate tolerance and the objective measurement of the barbiturate abstinence syndrome will be determined by chronic administration and subsequent withdrawal while daily motor control testing is maintained. Drugs will be given by gavage, but in some tolerance experiments they will be self-administered chronically by the schedule-induced polydipsia technique. Similar studies on motor control will be done on Rhesus monkeys and analyzed for tremor and dyskinesia by computer. In addition, chronic intravenous catheters will be used in some monkey experiments for barbiturate administration. Blood drug levels will be measured by gas chromatography to correlate these levels as well as to evaluate the current metabolic status with respect to tolerance development (drug elimination rates). Cross-abuse between phenobarbital and other agents will be ascertained by cronically loading rats with phenobarbital and noting their oral self-administration of other agents in a free-drinking situation and also where intake is continuously elevated by the polydipsia technique.